The lab (and Doctor, etc. ) are under contract with your insurance carrier. The contract requires them to accept payment for various services based on a rate schedule, and cannot bill the patient for the balance. If the doctor or whoever, is outside the network, then you probably will see a balance bill as they are not being held to that contract.
The doctor writes the difference off on his taxes as a loss, so is able to accept these terms without losing his shirt, as they generally grossly overcharge with this in mind anyway.
The problem is if someone doesn't have insurance, they get hit with the huge charges that were originally intended for milking this system. This is how medical insurance has turned our old system into a money machine and made the cost of medical care go sky high.
Of course when people complain about the cost of medicine in the U.S. they get a a cock and bull story about covering the cost of research, expensive equipment, etc. but this is what is really going on.
A similar system raises the cost of medicine as well, with the pharmaceutical companies lowering costs for many foreign companies, or their government subsidizing it; as the insurance scheme doesn't exist there.
